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A152141 a(n) is the least k such that 3*2^n*(2^k-1)-1 or 3*2^n*(2^k-1)+1 is prime ( or both primes). 2
1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 3, 1, 1, 2, 2, 2, 3, 2, 1, 5, 7, 2, 4, 4, 4, 8, 9, 3, 4, 4, 1, 11, 13, 2, 1, 9, 1, 3, 1, 5, 8, 1, 2, 1, 5, 6, 20, 15, 6, 7, 8, 3, 5, 13, 4, 1, 6, 43, 8, 4, 4, 7, 9, 2, 1, 2, 1, 2, 15, 42, 5, 10, 8, 18, 3, 10, 1, 8, 8, 7, 21, 2, 16, 19, 5, 4 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,10

LINKS

Pierre CAMI, Table of n, a(n) for n = 0..1600

EXAMPLE

3*2^0*(2^1-1)-1=2 is prime so a(0)=1;

3*2^1*(2^1-1)-1=5 is prime as well as 7 so a(1)=1;

3*2^2*(2^1-1)-1=23 is prime so a(2)=1.

PROG

(PARI) a(n) = my(k=1); while ((x=3*2^n*(2^k-1)-1) && !isprime(x) && !isprime(x+1), k++); k; \\ Michel Marcus, Sep 16 2019

CROSSREFS

Sequence in context: A255916 A107333 A161642 * A098505 A178395 A330958

Adjacent sequences:  A152138 A152139 A152140 * A152142 A152143 A152144

KEYWORD

nonn

AUTHOR

Pierre CAMI, Nov 26 2008

STATUS

approved

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Last modified August 4 13:49 EDT 2020. Contains 336201 sequences. (Running on oeis4.)